In its HTML5 support, IE9 will support playback of H.264 video as well as VP8 video when the user has installed a VP8 codec on Windows.

As we said at MIX recently, when it comes to HTML5, we’re all in. This level of commitment applies to the video codecs that IE9 will support as well. We are strongly committed to making sure that in IE9 you can safely view all types of content in all widely used formats. At the same time, Windows customers, developers, and site owners also want assurances that they are protected from IP rights issues when using IE9.

The key here, apparently, is that the user is responsible for installing the codec. So it's not as seamless as H.264 support.